Effect of stress concentrations on the fatigue behaviour of structural adhesives

With the increasing demand for lightweight components, structural adhesives have been playing a major role in the construction of multi-material joints with high stiffness and low weight [1]. Under cyclic loading conditions the fatigue lifetime of structural adhesive joints is considerably affected by stress concentrations arising from notches[2].The aim of the present research is to investigate the effect of stress concentrations on the fatigue behaviour of structural adhesives using numerical and experimental approaches with focus on the prediction of fatigue lifetime of notched specimens. To achieve this purpose, un-notched and internally as well as externally notched bulk specimens were investigated. The notches introduced to the samples were chosen to cover a wide range of stress concentrations, stress gradients and stress triaxialities. For the investigations, two commercial toughened epoxy structural adhesives with similar tensile strength were employed. The mechanical properties of these adhesives differ mainly in terms of plastic deformation prior to failure. Fracture surface analysis demonstrated the presence of stress whitening and voids close to the notch regions for both adhesives. These regions were correlated to higher values of stress concentration and stress triaxiality using FEA. The more ductile adhesive underwent widespread stress whitening prior to failure, whereas in the less ductile adhesive this whitening was more localised. Fatigue tests under constant amplitude and stress ratio of R = 0.1 revealed that fatigue strength is reduced in the presence of stress concentrations, especially in the high cycle fatigue range. This reduction is not only affected by the value of the maximum peak stress (stress concentration factor, kT), but also by the size of the highly stressed region(LHS). A model was proposed to explain the behaviour of SN curves of notched specimens in which the slope is controlled by the size of the highly stressed region and the stress concentration factor translates the position of the curve. The SN curves for crack initiation lifetime of both adhesives have indicated that most of the fatigue lifetime (60 to 95%) was spent on the crack initiation phase. Moreover, the crack initiation phase is shortened by an increasing stress concentration factor. These verity of this shortening is influenced by the mechanical properties of the adhesives(the most ductile adhesive was more sensitive) and by the level of applied stress. Experiments with externally notched specimens showed that the crack initiation causes a sudden increase of damage (i.e. reduction of stiffness) in the samples.